Magnetic resonance imaging scanner with molded fixed shims

ABSTRACT

A magnetic resonance imaging scanner includes a generally cylindrical main magnet assembly ( 10 ) that defines a cylinder axis ( 16 ). A first set of shims ( 60 ) are rigidly positioned inside the magnet assembly ( 10 ) at about a first distance (d 1 ) relative to the cylinder axis ( 16 ). A second set of shims ( 62 ) are rigidly positioned inside the main magnet assembly ( 10 ) at about a second distance (d 2 ) relative to the cylinder axis ( 16 ). The second distance (d 2 ) is different from the first distance (d 1 ). A generally cylindrical radio frequency coil ( 26 ) is arranged inside the main magnet assembly ( 10 ) at about a third distance (d 3 ) relative to the cylinder axis ( 16 ). A plurality of gradient coils ( 20 ) are arranged inside the main magnet assembly ( 10 ) at about a fourth distance (d 4 ) relative to the cylinder axis ( 16 ).

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application Ser.No. 60/474,637 filed May 30, 2003, which is incorporated herein byreference.

The following relates to the diagnostic imaging arts. It findsparticular application in magnetic resonance imaging using high-field,short-bore magnets, and will be described with particular referencethereto. However, it also finds application in other types of magneticresonance imaging scanners.

In magnetic resonance imaging, high main (B₀) magnetic fields arebeneficial. A high main magnetic field produces a stronger magneticresonance signal and consequently higher signal-to-noise ratios.Presently, main magnets of high resolution magnetic resonance imagingscanners provide main B₀ magnetic field strengths of about 1.5 tesla.Scanners with main magnets that produce fields higher than three teslaare projected.

For a fixed magnet length, however, it becomes increasingly difficult tomaintain magnetic field uniformity as the magnetic field strengthincreases. Hence, higher strength main magnets generally producecorrespondingly smaller usable imaging fields of view. The field of viewcan be enlarged by going to longer magnet bores. However, longer boresraise patient access, claustrophobia, and other issues.

Each scanner typically has custom magnetic field shimming using shims ofsteel or another magnetic material to compensate for magnetic fieldnon-uniformities. In one arrangement, the shims are mounted in traysthat are supported by a gradient coil former. The placement of fixedsteel closer to the imaging volume is complicated due to a large numberof other components that are also disposed within the magnet bore. Theseother components include whole-body radio frequency coils and optionallocal radio frequency coils, gradient coils for producing magnetic fieldgradients in the x-, y-, and z-directions, and shield coils forshielding nearby structures from the gradient fields. The radiofrequency coils in particular are placed inside the gradient coils andother metal structures so that the gradient coils do not affect theradio frequency coil. The radio frequency coils are preferably placed asclose as practicable to the imaging volume. Steel placed close to theimaging volume is more effective for stretching the field of view, andstrongly affects higher order magnetic field terms. Precise positioningof the fixed steel array is important for obtaining optimal performance.

The present invention contemplates an improved apparatus and method thatovercomes the aforementioned limitations and others.

According to one aspect, a magnetic resonance imaging scanner isdisclosed. A generally cylindrical main magnet assembly defines acylinder axis. A first set of shims is rigidly positioned inside themain magnet assembly at about a first distance relative to the cylinderaxis. A second set of shims is rigidly positioned inside the main magnetassembly at about a second distance relative to the cylinder axis. Thesecond distance is different from the first distance. A generallycylindrical radio frequency coil is arranged inside the main magnetassembly at about a third distance relative to the cylinder axis. Aplurality of gradient coils is arranged inside the main magnet assemblyat about a fourth distance relative to the cylinder axis.

According to another aspect, a method is provided of making a magneticresonance scanner. A first set of shims is rigidly positioned inside amain magnet assembly at about a first distance relative to a cylinderaxis of the main magnet assembly. A second set of shims is rigidlypositioned inside the main magnet assembly at about a second distancerelative to the cylinder axis. The second distance is different from thefirst distance. A generally cylindrical radio frequency coil is mountedinside the main magnet assembly at about a third distance relative tothe cylinder axis. A plurality of gradient coils are mounted inside themain magnet assembly at about a fourth distance relative to the cylinderaxis.

According to yet another aspect, a method is provided of magneticimaging with a magnetic resonance imaging scanner. The scanner includesa generally cylindrical main magnet assembly that defines a cylinderaxis, a first set of shims rigidly positioned inside the main magnetassembly at about a first distance relative to the cylinder axis, asecond set of shims rigidly positioned inside the main magnet assemblyat about a second distance relative to the cylinder axis that isdifferent from the first distance, a generally cylindrical radiofrequency coil arranged inside the main magnet assembly at about a thirddistance relative to the cylinder axis, and a plurality of gradientcoils arranged inside the main magnet assembly at about a fourthdistance relative to the cylinder axis. A substantially uniform magneticfield is generated within a field of view by cooperation of the mainmagnet assembly and the first and second sets of shims. A magneticresonance is excited within the field of view using one of the generallycylindrical radio frequency coil and another radio frequency coil. Themagnetic resonance is spatially encoded using magnetic field gradientsproduced by the plurality of gradient coils. The excited and spatiallyencoded magnetic resonance is detected using one of the generallycylindrical radio frequency coil and another radio frequency coil.

One advantage resides in reduced manufacturing time and effort, and astreamlined construction workflow.

Another advantage resides in arrangement of shims in symmetric packagesconfigured to have small net Z thrust forces, and radial forces that aredirected generally outward toward the package support structure.

Another advantage resides in improved precision and reduced tolerancesin placement of fixed steel in the magnet bore.

Another advantage resides in more efficient use of space within themagnet bore.

Yet another advantage resides in placement of steel close to the imagingvolume to enlarge the field of view. For a given amount of steel,placement of the steel close to the imaging volume is relatively moreeffective for affecting the magnetic field as compared with placement ofthe steel farther from the imaging volume.

Numerous additional advantages and benefits will become apparent tothose of ordinary skill in the art upon reading the following detaileddescription of the preferred embodiments.

The invention may take form in various components and arrangements ofcomponents, and in various process operations and arrangements ofprocess operations. The drawings are only for the purpose ofillustrating preferred embodiments and are not to be construed aslimiting the invention.

FIG. 1 diagrammatically shows a magnetic resonance imaging systemincluding a diagrammatic side sectional view of a magnetic resonanceimaging scanner.

FIG. 2 diagrammatically shows an end view of the magnetic resonanceimaging scanner of FIG. 1.

FIG. 3 shows a perspective view of a dielectric former of the magneticresonance imaging scanner with a birdcage radio frequency coil and fixedshim packs secured thereto.

FIG. 4 shows the perspective view of FIG. 3, with the fixed shim packssecured to the dielectric former. The birdcage radio frequency coil hasbeen removed in the view of FIG. 4.

FIG. 5 shows exemplary moldings containing fixed shim packs bonded to aseparately molded tray.

FIG. 6 shows a perspective sectional view of a portion of moldings andthe separately molded tray of FIG. 5, with shim packs encapsulated bythe moldings revealed by the section.

FIG. 7 shows a closer perspective view of one of the moldings of FIG. 5along with a portion of the separately molded tray.

FIG. 8 shows a perspective sectional view of the molding and trayportion of FIG. 7, with the encapsulated shim packs revealed by thesection.

With reference to FIGS. 1-3, a magnetic resonance imaging scanner (showndiagrammatically in cross-section) includes annular main magnet assembly10, which is preferably superconducting and surrounded by cold shields12. The main magnet assembly 10 typically includes active shieldingcoils. The main magnets 10 and the cold shields 12 define a magnet bore14 inside of which a patient or other imaging subject is placed forimaging. The main magnets 10 produce a spatially and temporally constantand uniform main magnetic field oriented along a longitudinal axis 16 ofthe bore 14. Instead of a superconducting magnet, a non-superconductingmagnet can be used. In a preferred embodiment, the main magnets 10define a short bore magnet of about 1.5 meters or less, and produce ahigh magnetic field of three tesla or higher in the magnet bore 14.However, the magnet can also be a longer bore magnet and/or produce alower strength magnetic field.

Magnetic field gradient coils 20 produce magnetic field gradients in thebore 14 for spatially encoding magnetic resonance signals, for producingmagnetization-spoiling field gradients, or the like. Preferably, themagnetic field gradient coils 20 include coils configured to producemagnetic field gradients in three orthogonal directions including thelongitudinal axial direction parallel to the main magnetic field. Forexample, the gradient coils 20 can include x-gradient coils, y-gradientcoils, and longitudinal z-gradient coils. Shield gradient coils 22shield the main magnets 10 and electrically conductive portions of thecold shields 12 and surrounding structures from the gradient magneticfields.

A generally cylindrical dielectric former 24 supports a radio frequencycoil assembly 26 that generates radio frequency pulses for excitingmagnetic resonances. The radio frequency coil assembly 26 also serves todetect magnetic resonance signals. Optionally, additional local radiofrequency coils or phased radio frequency coil arrays (not shown) areincluded for exciting and/or detecting magnetic resonances at localizedareas in the bore 14.

Gradient pulse amplifiers 30 deliver controlled electrical currents tothe magnetic field gradient coils 20 to produce selected magnetic fieldgradients. Magnetic field gradient controllers 32 control the gradientpulse amplifiers 30. A radio frequency transmitter 34, preferablydigital, applies radio frequency pulses or pulse packets to the radiofrequency coil assembly 26 to generate selected magnetic resonanceexcitations. A radio frequency receiver 36 also coupled to the radiofrequency coil assembly 26 receives magnetic resonance signals. If morethan one radio frequency coil is provided (such as a local coil orphased coil array), then different coils are optionally used for themagnetic resonance excitation and detection operations.

To acquire magnetic resonance imaging data of a subject, the subject isplaced inside the magnet bore 14, preferably at or near an isocenter ofthe main magnetic field. A sequence controller 40 communicates with thegradient controllers 32 and the radio frequency transmitter 34 toproduce selected transient or steady state magnetic resonanceconfigurations in the subject, to spatially encode such magneticresonances, to selectively spoil magnetic resonances, or otherwisegenerate selected magnetic resonance signals characteristic of thesubject. The generated magnetic resonance signals are detected by theradio frequency receiver 36, and stored in a k-space memory 44. Theimaging data is reconstructed by a reconstruction processor 46 toproduce an image representation that is stored in an image memory 48. Inone suitable embodiment the reconstruction processor 46 performs aninverse Fourier transform reconstruction.

The resultant image representation is processed by a video processor 50and displayed on a user interface 52, which is preferably a personalcomputer, workstation, or other type of computer. Rather than producinga video image, the image representation can be processed by a printerdriver and printed, transmitted over a computer network or the Internet,or the like. Preferably, the user interface 52 also allows a radiologistor other operator to communicate with the magnetic resonance sequencecontroller 40 to select magnetic resonance imaging sequences, modifyimaging sequences, execute imaging sequences, and so forth.

The main magnets 10 produce a substantially uniform magnetic field overan imaging field of view. To extend or stretch the field of view, abooster ring of fixed shims 60 is placed at about a radial distance d,relative to the cylinder axis 16 defined by the main magnet 10.Preferably, each fixed shim 60 includes one or more plates of steel oranother magnetic material that are secured together to define a shimpack 60. The amount of magnetic material in each shim pack 60corresponds to the number and thickness of secured steel plates. Whilethe use of shim packs is preferred to facilitate mass production ofshims of variable mass, it is also contemplated for each fixed shim toinclude a single unitary piece of steel or other magnetic materialhaving a selected mass that may in general be different for each fixedshim.

Additionally, adjustable shims 62 are preferably selectably arranged atabout a radial distance d₂ relative to the cylinder axis 16 to correctfor manufacturing imperfections or other non-uniformities of themagnetic field in the imaging field of view. Typically, the adjustableshims 62 are also steel plates or plates of another magnetic materialwhich are selectably placed into shim trays or other shim receptaclesafter manufacture of the main magnet 10 and with the booster ring ofshim packs 60 in place. The adjustable shims 62 are selectively insertedduring initial magnet calibration to improve magnetic field uniformitywithin the field of view. Optionally, larger diameter ferrous shim rings64 (shown diagrammatically in FIG. 1) are mounted in the primarygradient coil former.

The radio frequency coil assembly 26 is arranged at about a radialdistance d₃ relative to the cylinder axis 16, while the magnetic fieldgradient coils 20 are arranged at about a radial distance d₄ relative tothe cylinder axis 16. A metal radio frequency shield 66 shields thegradient coils 20 and other outer components from radio frequencysignals generated by the radio frequency coil 26. Typically, a boreliner (not shown) is provided at a radial distance smaller than theradial distances of the active components to prevent contact with theactive components of the scanner during imaging, and to improve theaesthetic appearance of the magnetic resonance imaging scanner 10.

In the illustrated embodiment, the first and second sets of shims 60,62, the radio frequency coil 26, and the gradient coils 20 have or arearranged about circular cross-sectional shapes or contours, and aresuitably described by distances d₁, d₂, d₃, d₄ which are radialdistances. However, those skilled in the art will recognize that one ormore of the first and second sets of shims, the radio frequency coil,and the gradient coils optionally have or are arranged about ellipticalor other cross-sectional shapes or contours rather than the illustratedexemplary circular cross-sectional contours. Such non-radialconfigurations are described by a suitable distance from the cylinderaxis. For example, a birdcage radio frequency coil having or arrangedabout an elliptical contour is suitably described by a distance from thecylinder axis to the coil along a major ellipse axis, a distance fromthe cylinder axis to the coil along a minor ellipse axis, or distancefrom the cylinder axis to the coil along an intermediate axis of theellipse.

In a preferred embodiment, the distance d₁ at which the fixed shim packs60 of the booster ring are disposed is smaller than the distance d₂ atwhich the adjustable shims 62 are selectably arranged. Shims placedrelatively closer to the field of view have a relatively stronger effecton the higher order magnetic field harmonics, and so the preferredsmaller distance d₁ causes the fixed shim packs 60 to adjust the higherorder harmonics to effectively extend or stretch the field of view, thatis, the region of substantially uniform magnetic field. The adjustableshims 62 are selectably arranged at the larger distance d₂ where theyhave a greater effect on lower order magnetic field terms.

To efficiently use space near the imaging volume within the bore 14, ina preferred arrangement the fixed shim packs 60 are disposed on the samedielectric former 24 on which the radio frequency coil assembly 26 isdisposed. In this arrangement, the distance d₁ of the fixed shim packs60 substantially equals the distance d₃ of the radio frequency coilassembly 26. The distance d₄ of the gradient coils 20 is preferablysubstantially larger than the distances d₁, d₃. In one preferredembodiment, the adjustable shims 62 are selectably arranged outside ofthe primary gradient coils 20; that is, the distance d₂ is greater thanthe distance d₄. However, the adjustable shims 62 can also be placedinside the primary gradient coils 20; that is, the distance d₄ of thegradient coils can be greater than or equal to the distance d₂ of theadjustable shims.

The fixed shim packs 60 are arranged between rungs 70 of the radiofrequency coil assembly 26, which in a preferred embodiment is abirdcage coil. In the exemplary illustrated embodiment, the birdcagecoil assembly 26 has sixteen rungs 70 arranged generally parallel to thecylinder axis 16, and the booster ring includes sixteen rows of fixedshim packs 60 arranged at radial positions between the radio frequencycoil rungs 70. As best seen diagrammatically in FIG. 2, the fixed shimpacks 60 are preferably arranged in a radially symmetric fashionrespective to the cylinder axis 16. The fixed shim packs 60 arepreferably also arranged with a bilateral symmetry about a longitudinalsymmetry plane 72 (shown in FIG. 1) of the bore 14. The radial andbilateral symmetries promote controlled and symmetric modification ofhigher order terms of the main (B₀) magnetic field. In contrast, theadjustable shims 62 may optionally lack one or both of radial symmetryabout the cylinder axis 16 and bilateral symmetry about the longitudinalsymmetry plane 72.

By placing the fixed shim packs 60 of the booster ring close to theimaging volume, substantial stretching of the imaging volume isachievable with a limited amount of steel or other magnetic material.However, this close placement also leads to a high sensitivity toprecise placement of the fixed shim packs 60. Moreover, the shim packs60 generally experience substantial magnetic forces from the main B₀magnetic field and from magnetic field gradients produced by thegradient coils 20. Hence, the fixed shim packs 60 should be preciselyand rigidly secured to the dielectric former 24. In a preferredembodiment, the fixed shim packs 60 are secured to the dielectric former24 through the use of plastic encapsulation.

With reference to FIGS. 4 and 5, an exemplary molded strip 80 of twelvefixed shim packs 60 ₁, 60 ₂, 60 ₃, 60 ₄, 60 ₅, 60 ₆, 60 ₇, 60 ₈, 60 ₉,60 ₁₀, 60 ₁₁, 60 ₁₂ is shown. FIG. 6 shows a cross-sectional view of aportion of the molded strip 80 including the first seven fixed shimpacks 60 ₁, 60 ₂, 60 ₃, 60 ₄, 60 ₅, 60 ₆, 60 ₇. In the illustratedembodiment, between one and three fixed shim packs is encapsulated ineach molding. Thus, a molding 82 includes the shim packs 60 ₁, 60 ₂, 60₃; a molding 84 includes the shim packs 60 ₄, 60 ₅; a molding 86includes the shim pack 60 ₆; a molding 88 includes the shim pack 60 ₇; amolding 90 includes the shim packs 60 ₈, 60 ₉; and a molding 92 includesthe shim packs 60 ₁₀, 60 ₁₁, 60 ₁₂. Although between one and three shimpacks are included in each molded pack of the illustrated embodiment, itis contemplated to include more than three shim packs in anencapsulation molding.

The moldings 82, 84, 86, 88, 90, 92 are secured to a separately moldedtray 96. In a preferred embodiment, the moldings 82, 84, 86, 88, 90, 92are secured by ultrasonic bonding to the separately molded tray 96. Eachmolding 82, 84, 86, 88, 90, 92 included mating extensions 100 (see FIGS.6 and 8) that mate with and ultrasonically bond with correspondingsurfaces of the separately molded tray 96. Additionally, the separatelymolded tray 96 includes extended sides 102 (see FIGS. 5-7) that furthersupport and position the moldings 82, 84, 86, 88, 90, 92. Althoughultrasonic bonding is preferred, other bonding techniques, such aspotting or fastening by mechanical fasteners, are also contemplated forsecuring the moldings to the tray. The separately molded tray 96 issecured to the generally cylindrical dielectric former 22 by fasteners,ultrasonic bonding, or the like.

With particular reference to FIGS. 7 and 8, the exemplary molding 82encapsulates fixed shim packs 60 ₁, 60 ₂, 60 ₃. The plates of steel orother magnetic material are riveted together by rivets 104 to form theshim packs 60 ₁, 60 ₂, 60 ₃ prior to encapsulation. The number ofmagnetic plates in each shim pack may in general be different. Forexample, FIG. 8 shows that the shim packs 60 ₁, 60 ₂ contain moremagnetic plates than the shim pack 60 ₃. In a preferred embodiment, theshim packs 60 ₁, 60 ₂, 60 ₃ are positioned in a mold using aspring-biased pin or other molding fixture, and the plastic molding 82is molded around the steel by injection molding, resin transfer molding(RTM), compression molding, or another suitable technique. The moldingfixture is removed through openings 106 in the molding 82.

A consideration in selecting the molding material is thermal matchingwith the steel or other magnetic material. A large thermal mismatch cancause cracking or other damage during setting of the molding 82. Theinventors have performed thermal simulations and have determined thatthe ultimate strain of the encapsulation material is defined accordingto:ε_(ult)≧(Δα·ΔT)F.S.  (1),whereΔα=α_(encapsulation)−α_(shim)  (2),ΔT=T _(g) −T _(min)  (3),ε_(ult) is the ultimate strain of the encapsulation material,α_(encapsulation) is the coefficient of thermal expansion of theencapsulation material, α_(shim) is the coefficient of thermal expansionof the shim material, T_(g) is the glass transition temperature of theencapsulation material, T_(min) is a minimum use temperature, and F.S.is a factor of safety. In a preferred embodiment, the molding 82 is madeof Ultem® (a polyetherimide thermoplastic of General Electric with athermal coefficient of expansion of 5.6×10⁻⁵ mm/mm·°C.). Ultem® has arelatively large ultimate strain of about 60%, an advantageous U.L.rating, and high strength, toughness, and stability. However, otherthermal plastics can be selected in accordance with Equations (1)-(3).When using the preferred Ultem® encapsulation material, the molding 82is advantageously annealed for between four and eight hours toaccelerate stress relief. Alternatively, stress relief occurs withoutannealing over a period of about one to two weeks. In one embodiment,the corners of the fixed shim packs 60 ₁, 60 ₂, 60 ₃ are rounded toreduce stresses at the comers.

With particular reference returning to FIGS. 5 and 6, the moldings 82,84, 86, 88, 90, 92 are also designed to withstand substantial forcesexerted on the fixed shim packs 60 ₁, 60 ₂, 60 ₃, 60 ₄, 60 ₅, 60 ₆, 60₇, 60 ₈, 60 ₉, 60 ₁₀, 60 ₁₁, 60 ₁₂ by the main B₀ magnetic field and bymagnetic field gradients. One technique used to reduce these forces isforce balancing within the moldings. That is, two or three shim packs ina single molding are preferably force balanced such that they exert acompressive force on molding material between the shim packs. The Ultem®molding material is more resistant to compressive stress than tensilestress. Rather than using a plurality of moldings 82, 84, 86, 88, 90, 92that are bonded to a separately molded tray 96, it is contemplated tomake a single molding that encompasses the fixed shim packs 60 ₁, 60 ₂,60 ₃, 60 ₄, 60 ₅, 60 ₆, 60 ₇, 60 ₈, 60 ₉, 60 ₁₀, 60 ₁₁, 60 ₁₂.

The invention has been described with reference to the preferredembodiments. Obviously, modifications and alterations will occur toothers upon reading and understanding the preceding detaileddescription. It is intended that the invention be construed as includingall such modifications and alterations insofar as they come within thescope of the appended claims or the equivalents thereof.

1. A magnetic resonance imaging scanner including: a generallycylindrical main magnet assembly that defines a cylinder axis; a firstset of shims rigidly positioned inside the main magnet assembly at abouta first distance relative to the cylinder axis, the first set of shimsincluding a generally cylindrical dielectric former, packets of magneticmaterial disposed on the generally cylindrical dielectric former, andplastic encapsulation encapsulating the packets of magnetic material,wherein the plastic encapsulation includes openings through whichmolding fixtures are removed; a second set of shims rigidly positionedinside the main magnet assembly at about a second distance relative tothe cylinder axis, the second distance being different from the firstdistance; a generally cylindrical radio frequency coil arranged insidethe main magnet assembly at about a third distance relative to thecylinder axis; and a plurality of gradient coils arranged inside themain magnet assembly at about a fourth distance relative to the cylinderaxis.
 2. The magnetic resonance imaging scanner as set forth in claim 1,wherein the first distance equals the third distance.
 3. The magneticresonance imaging scanner as set forth in claim 1, wherein the first setof shims has a radial symmetry respective to the cylinder axis.
 4. Themagnetic resonance imaging scanner as set forth in claim 3, wherein thefirst set of shims has a bilateral symmetry respective to a longitudinalsymmetry plane that is perpendicular to the cylinder axis.
 5. Themagnetic resonance imaging scanner as set forth in claim 4, wherein thesecond set of shims is asymmetric about at least one of the cylinderaxis and the longitudinal plane of symmetry.
 6. The magnetic resonanceimaging scanner as set forth in claim 1, wherein the plasticencapsulation includes: a separately molded trays each securing one ormore of the packets of magnetic material to the generally cylindricaldielectric former.
 7. The magnetic resonance imaging scanner as setforth in claim 1, wherein the plastic encapsulation has a coefficient ofthermal expansion α_(encapsulation) such that ε_(ult)≧(Δα·ΔT)F.S., whereΔα=α_(encapsulation)−α_(shim), ΔT=T_(g)−T_(min), ε_(ult) is the ultimatestrain of the encapsulation material, α_(encapsulation), is thecoefficient of thermal expansion of the encapsulation material, α_(shim)is the coefficient of thermal expansion of the shim material, T_(g) isthe glass transition temperature of the encapsulation material, T_(min)is a minimum use temperature, and F.S. is a factor of safety.
 8. Themagnetic resonance imaging scanner as set forth in claim 1, wherein theplastic encapsulation is made of a polyetherimide thermoplastic.
 9. Themagnetic resonance imaging scanner as set forth in claim 1, wherein thepackets of magnetic material each include: one or more steel platessecured together by at least one fastener.
 10. The magnetic resonanceimaging scanner as set forth in claim 9, wherein the steel plates haverounded edges to reduce stress between the steel plates and the plasticencapulation.
 11. The magnetic resonance imaging scanner as set forth inclaim 1, wherein the first distance equals the third distance, and theradio frequency coil rungs are secured to the generally cylindricaldielectric former.
 12. A magnetic resonance imaging scanner including: agenerally cylindrical main magnet assembly that defines a cylinder axis;a first set of shims rigidly positioned inside the main magnet assemblyat about a first distance relative to the cylinder axis, the first setof shims including a generally cylindrical dielectric former, packets ofmagnetic material disposed on the generally cylindrical dielectricformer, and plastic encapsulation encapsulating the packets of magneticmaterial; a second set of shims rigidly positioned inside the mainmagnet assembly at about a second distance relative to the cylinderaxis, the second distance being different from the first distance; agenerally cylindrical radio frequency coil arranged inside the mainmagnet assembly at about a third distance relative to the cylinder axis;and a plurality of gradient coils arranged inside the main magnetassembly at about a fourth distance relative to the cylinder axis;wherein the radio frequency coil includes a plurality of rungs arrangedgenerally parallel to the cylinder axis, wherein the packets of magneticmaterial are disposed at radial positions between the rungs.
 13. Themagnetic resonance imaging scanner as set forth in claim 12, wherein thefirst distance equals the third distance, and the radio frequency coilrungs are secured to the generally cylindrical dielectric former. 14.The magnetic resonance imaging scanner as set forth in claim 12, furtherincluding: shim rings arranged inside the main magnet assembly at adistance larger than the first distance and less than the seconddistance, the shim rings being arranged symmetrically relative to alongitudinal plane of symmetry.
 15. The magnetic resonance imagingscanner as set forth in claim 12, wherein the first distance equals thethird distance.
 16. The magnetic resonance imaging scanner as set forthin claim 12, wherein the plastic encapsulation has a coefficient ofthermal expansion α_(encapsulation) such that ε_(ult)≧(Δα·ΔT)F.S., whereΔα=α_(encapsulation)−α_(shim), ΔT=T_(g)−T_(min), ε_(ult) is the ultimatestrain of the encapsulation material, α_(encapsulation) is thecoefficient of thermal expansion of the encapsulation material, α_(shim)is the coefficient of thermal expansion of the shim material, T_(g) isthe glass transition temperature of the encapsulation material, T_(min)is a minimum use temperature, and F.S. is a factor of safety.
 17. Themagnetic resonance imaging scanner as set forth in claim 12, wherein thepackets of magnetic material each include one or more steel platessecured together by at least one fastener, and the steel plates haverounded edges to reduce stress between the steel plates and the plasticencapulation.
 18. The magnetic resonance imaging scanner as set forth inclaim 12, wherein the first set of shims has a radial symmetryrespective to the cylinder axis.
 19. The magnetic resonance imagingscanner as set forth in claim 12, wherein the plastic encapsulation ismade of a polyetherimide thermoplastic.
 20. A method of making amagnetic resonance scanner, the method including: rigidly positioning afirst set of shims inside a main magnet assembly at about a firstdistance relative to a cylinder axis of the main magnet assembly bymolding a plastic material around the first shims and bonding the moldedplastic material to a generally cylindrical former; rigidly positioninga second set of shims inside the main magnet assembly at about a seconddistance relative to the cylinder axis, the second distance beingdifferent from the first distance; mounting a generally cylindricalradio frequency coil inside the main magnet assembly at about a thirddistance relative to the cylinder axis; and mounting a plurality ofgradient coils inside the main magnet assembly at about a fourthdistance relative to the cylinder axis.
 21. The method as set forth inclaim 20, wherein the molding includes: fastening the first shims in aninjection mold using at least one fastener; and injection molding theplastic material around the first shims.
 22. The method as set forth inclaim 21, wherein the molding further includes: after the injectionmolding, removing the at least one fastener.
 23. The method as set forthin claim 21, wherein the molding further includes: prior to thefastening of each first shim in the injection mold, binding a selectednumber of metal sheets together to define the first shim.
 24. The methodas set forth in claim 20, wherein the bonding of the molded plasticmaterial to the generally cylindrical former includes: ultrasonicallybonding the molded plastic material to a separately molded tray; andfastening the separately molded tray to the dielectric former.
 25. Themethod as set forth in claim 20, wherein the molding of a plasticmaterial around the first set of shims produces a plurality of moldingseach including at least one shim of the first set of shims, and thebonding of the molded plastic material to the generally cylindricalformer includes: bonding the moldings at radially spaced apart positionsaround the generally cylindrical former.
 26. The method as set forth inclaim 25, further including: securing rungs of the radio frequency coilto the generally cylindrical former in radial gaps between the moldings.27. The method as set forth in claim 20, wherein the molding of theplastic material around the shim includes: molding the plastic materialaround the shim; and thermally annealing to relieve stress between theshim and the plastic material.